Microelectronic device with a memory element utilizing stacked vertical devices

ABSTRACT

An embodiment may include a method of forming a microelectronic device. The method may include forming a pair of transistors stacked vertically and connected in series, each of the pair of transistors are of the same type. The method may include forming a memory element including a first inverter containing a first inverter transistor and an access transistor. The first inverter transistor is connected to a power supply rail. The access transistor is connected to a bitline. The first inverter transistor is a first transistor of the pair of vertically stacked transistors and the access transistor is a second transistor of the pair of vertically stacked transistors. The pair of transistors are arranged substantially perpendicular to the plurality of layers.

BACKGROUND

The current disclosure relates to an integrated circuit and a method formanufacturing thereof.

Most of the time, integrated circuits include multiple adjacent layerswhich are deposited and structured one after the other. Power supplyrails are provided to supply power to the cells of the integratedcircuit. Cells of the integrated circuit may include static randomaccess memory (SRAM) cells for storing data and logic cells forperforming combinatorial logic functions, such as using NAND gates andNOR gates. Signal lines are provided for transmitting data, for example,the result of a Boolean operation or data (to be) stored in a cell(e.g., a SRAM cell). The cells of the integrated circuit include fieldeffect transistors (FETs), each including a drain region, a sourceregion, a channel region and a gate region. Typically, the FETs areformed in the lowest layers of the integrated circuit. The layerscomprising the signal lines and the power supply rails are provided inlayers above the FETs.

Vertical interconnect access contacts (VIAs) are provided to connect thedrain regions, the source regions and the gates of the FETs to thesignal lines and the power supply rails. Each VIA leads to contactcapacitances and resistances at the interface to the respective regionsof the FETs, signal lines and power supply rails. Moreover, each VIAenlarges the surface area required for the manufacture of the integratedcircuit. Therefore, there may be a need for reducing the amount of VIAsrequired for the manufacture of an integrated circuit.

Moreover, the individual VIAs may require alignment with the sourceregions and the gates of the FETs, and the signal lines and the powersupply rails. There may be a need for facilitating the manufacture ofvertical structures connecting conductors extending in different layersof the integrated circuit.

SUMMARY

Shortcomings of the prior art are overcome, and additional advantagesare provided through the provision of the integrated circuit and themethod according to the independent claims. Advantageous embodiments aredescribed in the dependent claims.

According to an embodiment, an integrated circuit is provided thatincludes multiple adjacent layers, where the integrated circuit includesa plurality of vertical structures, where the vertical structures extendin a vertical direction that is substantially perpendicular to thelayers of the integrated circuit, where the vertical structures includesat least one vertical connection element and at least one verticalchannel region, and where the vertical structures are arranged abovenodes of a virtual two-dimensional regular grid. There may be nodes ofthe grid, on which no vertical structure is provided. The regularity ofthe grid may facilitate manufacturing the integrated circuit.

According an embodiment, an integrated circuit is provided. Theintegrated circuit includes multiple adjacent layers that include astack of one or more field effect transistors (FETs) (e.g., a first FETand a second FET). The FETs each include a channel region. The channelregions of the FETs of the stack are electrically connected in series.The stack extends in a vertical direction that is substantiallyperpendicular to the layers (e.g., layers of the integrated circuit).The first FET and the second FET are of the same type.

According to an embodiment, the two opposed ends of the stack correspondto a power support end and a signal end. The power support end isconnected to a power supply rail of the integrated circuit. The signalend is connected to signal lines of the integrated circuit.

According to an embodiment, the FETs of the stack are of the same type,wherein the channel regions of the FETs of the stack are electricallyconnected in series, wherein the two opposed ends of the stackcorrespond to a power support end and a signal end, and wherein thepower support end is connected to a power supply rail of the integratedcircuit.

According to an embodiment, several cell types (i.e., cells performingBoolean operations or storing data) within an integrated circuit requiretwo FETs of the same type connected in series. A stack of two FETsextending in a vertical direction may facilitate wiring and routingwithin and between the functional cells. This may allow for a highercell density within the integrated circuit.

According to an embodiment, the power supply rail may be provided as aburied conductive layer in a substrate of the integrated circuit. Inparticular, the power supply rail may be formed as a heavily dopedsemiconductor region. The heavily doped semiconductor region may beformed by an ion implant process. Alternatively, the heavily dopedsemiconductor region may be formed by a diffusion process. The lowresistive power supply rail may be formed by different process options(e.g., compound methods).

According to an embodiment, the channel regions of the FETs of the stackare provided in layers between the layer comprising the power supplyrail of the integrated circuit and layers comprising signal lines of theintegrated circuit.

Separating the layer(s) comprising signal lines and the layer(s)comprising the power supply rail(s) may substantially reduce the routingcomplexity of the integrated circuit. In particular, the number of VIAcontacts and wire lengths may be reduced. This may allow for a higherdevice density of the integrated circuit and/or a reduced powerconsumption of the integrated circuit.

According to an embodiment, the stack may be formed by epitaxy on theheavily doped semiconductor region forming the power supply rail. Thedopant concentration may be changed during epitaxy.

According to an embodiment, the channel regions of the FETs of the stackare horizontal channel regions in which charge carriers move in adirection parallel to the layers of the integrated circuit in aconductive state of the FETs. The stack may include two classical planarFETs one above the other, where the source region of one of the oneclassical planar FETs is electrically connected to the drain region ofthe other classical planar FET. Alternatively, the stack may include twoFinFETs in series with a source-drain connection. The fins of theFinFETs may be provided parallel or orthogonal to each other.

According to an embodiment, the first FET and/or the second FET includesmore than one vertical channel region. Providing more than one verticalchannel may allow for an easier adaptation of the FET to differentelectrical requirements. The vertical channel regions may havesubstantially the same dimensions to improve process stability formanufacturing the integrated circuit. Substantially the same dimensionmay refer to dimensions which only fluctuate due to unavoidabledeviations during the processing of the integrated circuit. Thecharacteristic of a FET of the integrated circuit may be tuned byselecting the number of vertical channel regions. For example, a FET mayinclude two times the amount of vertical channel regions of another FETif the FET has to provide twice the current of the other FET. Hence, thedimensions of the FETs may be quantized.

According to an embodiment, the vertical channel region has a crosssection having a longitudinal axis and a transversal axis, wherein thelongitudinal axis and the transversal axis have substantially the samelength. For example, the vertical channel region may have a circularcross section. A circular cross section may result from exposure using amask with quadratic features due to exposure artifacts at the corners ofthe quadrat. In an embodiment, the vertical channel region may have ahexagonal cross section. For example, the vertical channel region may beformed using a semiconductor having a hexagonal crystal structure.Moreover, in an embodiment, the vertical channel region may have atriangular cross section. The triangle may be an equilateral triangle.Using a vertical channel region with a cross section having alongitudinal axis and a transversal axis of substantially the samelength may result in FETs having a particularly narrow channel.

According to an embodiment, the vertical channel region may be formed byepitaxial growth in a direction vertical to the layers. Epitaxial growthmay allow for a precise definition of the length (e.g., vertical length)of the vertical channel region. In particular, the length of verticalchannel regions of several FETs provided in the same layer of theintegrated circuit may be formed to be substantially the same.Deviations in channel lengths induced by patterning processes, forexample using extreme ultra violet (EUV) lithography, may be avoided.

According to an embodiment, the longitudinal axis is longer than thetransversal axis. For example, the vertical channel region may have arectangular or elliptical cross section. Vertical channel regions havingan elongated cross section with a longer longitudinal axis and a shortertransversal axis, or vice versa, may allow for a wider channel for agiven cross section area.

According to an embodiment, the longitudinal axis of the elongated crosssection is substantially perpendicular to a longitudinal direction ofthe power supply rail. Such an arrangement may optimize usage of thespace vertically above the power supply rail while providing narrowquantized channels.

According to an embodiment, the stack includes a common nanowire. Thecommon nanowire includes the channel regions of the multiple FETs. Aregion located between the channel region of the first FET and thechannel region of the second FET may at the same time form thesource/drain region of the first FET and the drain/source region of thesecond FET.

According to an embodiment, the first FET and/or the second FET is agate-all-around FET. Providing a gate-all-around embodiment of thechannel region of the first/second FET may improve the electricalcharacteristics of the FET. Boundary effects at boundaries of the gateextending parallel to the channel region may be avoided.

According to an embodiment, the integrated circuit includes a secondstack of two FETs. The two FETs of the second stack are of the sametype. The two FETs of the second stack have a different type than thefirst FET and the second FET. A power end of the second stack isconnected to another power supply rail.

According to an embodiment, the integrated circuit includes a SRAM cell.The SRAM cell includes a memory element and an access FET. The memoryelement includes a first inverter comprising a first inverter FETconnected to a power supply rail. The first inverter FET is the firstFET of the stack. The access FET is the second FET of the stack. Theaccess FET is electrically connected to a bitline.

According to an embodiment, the proposed SRAM cell may require less VIAsthan a classical SRAM cell. Therefore, the proposed SRAM cell may have asmaller footprint and the SRAM cell density of the integrated circuitmay be increased. Furthermore, wires connecting elements of the proposedSRAM cell may be shorter, thus, reducing power consumption of theintegrated circuit due to wire capacitances. The bitline may beconsidered as a signal line of the integrated circuit.

According to an embodiment, the integrated circuit includes twomirror-symmetrical SRAM cells. Providing mirror-symmetrical SRAM cellsmay allow for an increased density of the SRAM cells of the integratedcircuit.

According to an embodiment, the mirror-symmetrical SRAM cells share thepower supply rail. The mirrored-symmetrical SRAM cells sharing the powersupply rail may allow for the provision of a wider power supply rail.Thus, voltage fluctuations along the power supply rail may be bettermitigated.

According to an embodiment, the integrated circuit includes a logiccell. The logic cell includes the stack and a second pair of FETs. Thesecond pair of FETs is of a different type than the first FET and thesecond FET. The two FETs of the second pair are electrically arranged inparallel. The two FETs of the second pair form vertical structuresdifferent from the stack.

According to an embodiment, a gate of one FET of the stack and a gate ofone FET of the second pair are connected to each other and correspond toa first input of the logic cell. The gate of the FET of the stack andthe gate of the one FET of the second pair extend in the same layerwithin the layered structure of the integrated circuit. In anembodiment, the gate of one FET of the stack and the gate of the one FETof the second pair may be formed as one piece.

According to an embodiment, the gate of one FET of the stack and thegate of one FET of the second pair are connected to each other andcorrespond to a second input of the logic cell. The gate of the one FETof the stack and the gate of the one FET of the second pair extend indifferent layers of the layer structure of the integrated circuit.

According to an embodiment, the power supply rails extend in at leastone layer located at one first end of the stack and/or the verticalstructure. Signal interconnect conductors extend in a layer located at asecond, opposite end of the stack or vertical structure.

According to an embodiment, the logic cell is a NAND gate or a NOR gate.

According to an embodiment, the integrated circuit includes more thanone cell selected from the group of SRAM cells and logic cells describedherein, wherein the selected cells share at least one power supply rail.

According to an embodiment, the selected cells share one of the VDDpower supply rail and the ground power supply rail and use a differentpower supply rail for the further potential.

According to an embodiment, the selected cells share both power supplyrails.

According to an embodiment, there may be nodes of the grid, on which novertical structure is provided. The regularity of the grid mayfacilitate manufacturing the integrated circuit.

According to an embodiment, a longitudinal spacing between two nodes ofthe grid in a longitudinal direction is equal to a transversal spacingbetween two nodes of the grid in a transversal direction, wherein thelongitudinal direction is perpendicular to the transversal direction. Inparticular, the grid may correspond to a checker-board.

According to an embodiment, a longitudinal spacing between two nodes ofthe grid in a longitudinal direction is different from the transversalspacing between two nodes of the grid in a transversal direction,wherein the longitudinal direction is perpendicular to the transversaldirection. Doing so allows for a better insulation between two powersupply rails providing different voltages if the power supply railsextend along a transversal or longitudinal direction of the grid. Inparticular, the differing longitudinal and transversal spacing mayreduce the amount of leakage currents and/or unintentional capacitances.

According to an embodiment, the grid has a hexagonal pattern. Ahexagonal pattern may allow for a particular dense provision of thevertical structures.

An embodiment may include a microelectronic device. The microelectronicdevice may include a pair of transistors stacked vertically andconnected in series. Each of the pair of transistors are of the sametype. The microelectronic device may include a memory element includingan inverter containing a first inverter transistor and an accesstransistor. The first inverter transistor is connected to a power supplyrail. The access transistor is connected to a bitline. The firstinverter transistor is a first transistor of the pair of verticallystacked transistors and the access transistor is a second transistor ofthe pair of vertically stacked transistors. The pair of transistors arearranged substantially perpendicular to the plurality of layers.

An embodiment may include a microelectronic device. The microelectronicdevice may include a pair of transistors stacked vertically andconnected in series. Each of the pair of transistors are of the sametype. The microelectronic device may include a memory element includingan inverter and a cross-coupling inverter. The inverter includes a firstinverter transistor connected to a power supply rail and a secondinverter transistor connected to a further power supply rail. The secondinverter transistor is a different type than the first invertertransistor. The first inverter transistor is a first transistor of thepair of vertically stacked transistors. A gate of the first invertertransistor and a gate of the second inverter transistor are electricallyconnected to each other by a single conductive element extending withina single layer of the microelectronic device. The first invertertransistor and the second inverter transistor are substantiallyperpendicular to the plurality of layers. The microelectronic device mayinclude an access transistor. The access transistor is connected to abitline. A gate of the access transistor is connected to a wordline. Theaccess transistor is a second transistor of the pair of verticallystacked.

An embodiment may include a method of forming a microelectronic device.The method may include forming a pair of transistors stacked verticallyand connected in series. Each of the pair of transistors are of the sametype. The method may include forming a memory element including aninverter containing a first inverter transistor and an accesstransistor. The first inverter transistor is connected to a power supplyrail. The access transistor is connected to a bitline. The firstinverter transistor is a first transistor of the pair of verticallystacked transistors and the access transistor is a second transistor ofthe pair of vertically stacked transistors. The pair of transistors arearranged substantially perpendicular to the plurality of layers.

An embodiment may include a method of forming a microelectronic device.The method may include forming a pair of transistors stacked verticallyand connected in series. Each of the pair of transistors are of the sametype. The method may include forming a memory element including aninverter and a cross-coupling inverter. The first inverter includes afirst inverter transistor connected to a power supply rail and a secondinverter transistor connected to a further power supply rail. The secondinverter transistor is a different type than the first invertertransistor. The first inverter transistor is a first transistor of thepair of vertically stacked transistors. The first inverter transistorand the second inverter transistor are substantially perpendicular tothe plurality of layers. A gate of the first inverter transistor and agate of the second inverter transistor are electrically connected toeach other by a single conductive element extending within a singlelayer of the microelectronic device. The method may include forming anaccess transistor. The access transistor is connected to a bitline and agate of the access transistor is connected to a wordline. The accesstransistor is a second transistor of the pair of vertically stacked.

In particular, there is provided a method for manufacturing anintegrated circuit including multiple adjacent layers with any one ofthe features mentioned before. Concerning advantageous effects of theproposed method, reference is made to the description of the embodimentsof the integrated circuit described in this disclosure.

Additional features and advantages are realized through the techniquesdisclosed herein. Other embodiments and aspects of the invention aredescribed in detail herein and are considered a part of the claimedaspects.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more aspects of the present invention are particularly pointedout and distinctly claimed as examples in the claims at the conclusionof the specification. The foregoing and other objects, features, andadvantages of the invention are apparent through the following detaileddescription taken in conjunction with the accompanying drawings inwhich:

FIGS. 1a-6a show circuit diagrams with elements of a NAND gate;

FIGS. 1b-6b show the physical structure of the elements shown in FIGS.1a -6 a;

FIG. 7a shows a circuit diagram of a balanced NAND gate;

FIG. 7b shows the physical structure of the balanced NAND gate of FIG. 7a;

FIG. 7c shows vertical structures on nodes of a grid;

FIG. 7d further illustrates the grid of FIG. 7 c;

FIGS. 8a-13a show a circuit diagrams with element of a NOR gate;

FIGS. 8b-13b show the physical structure of the elements shown in FIGS.8a -13 a;

FIGS. 14a-20a show circuit diagrams with elements of a SRAM cell;

FIGS. 14b-20b show the physical structure of the elements shown in FIGS.14a -20 a;

FIGS. 21-22 show the physical structure of a first array of SRAM cells;

FIG. 23 shows the physical structure of a second array of SRAM cells;

FIG. 24 shows the physical structure of a third array of SRAM cells;

FIG. 25 shows vertical structures on nodes of a grid; and

FIG. 26 shows nodes of another grid.

DETAILED DESCRIPTION

Detailed embodiments of the claimed structures and methods are disclosedherein; however, it can be understood that the disclosed embodiments aremerely illustrative of the claimed structures and methods that may beembodied in various forms. This invention may, however, be embodied inmany different forms and should not be construed as limited to theexemplary embodiments set forth herein. Rather, these exemplaryembodiments are provided so that this disclosure will be thorough andcomplete and will fully convey the scope of this invention to thoseskilled in the art. In the description, details of well-known featuresand techniques may be omitted to avoid unnecessarily obscuring thepresented embodiments.

References in the specification to “one embodiment”, “an embodiment”,“an example embodiment”, etc., indicate that the embodiment describedmay include a particular feature, structure, or characteristic, butevery embodiment may not necessarily include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to affect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed.

For purposes of the description hereinafter, the terms “upper”, “lower”,“right”, “left”, “vertical”, “horizontal”, “top”, “bottom”, andderivatives thereof shall relate to the disclosed structures andmethods, as oriented in the drawing figures. The terms “overlying”,“atop”, “on top”, “positioned on” or “positioned atop” mean that a firstelement, such as a first structure, is present on a second element, suchas a second structure, wherein intervening elements, such as aninterface structure may be present between the first element and thesecond element. The term “direct contact” means that a first element,such as a first structure, and a second element, such as a secondstructure, are connected without any intermediary conducting, insulatingor semiconductor layers at the interface of the two elements.

In the interest of not obscuring the presentation of embodiments of thepresent invention, in the following detailed description, someprocessing steps or operations that are known in the art may have beencombined together for presentation and for illustration purposes and insome instances may have not been described in detail. In otherinstances, some processing steps or operations that are known in the artmay not be described at all. It should be understood that the followingdescription is rather focused on the distinctive features or elements ofvarious embodiments of the present invention. Furthermore, it should benoted that while this description may refer to some components of thestructure in the singular tense, more than one component may be depictedthroughout the figures.

The terms substantially, substantially similar, about, or any other termdenoting functionally equivalent similarities refer to instances inwhich the difference in length, height, or orientation convey nopractical difference between the definite recitation (e.g. the phrasesans the substantially similar term), and the substantially similarvariations. In an embodiment, substantial (and its derivatives) denote adifference by a generally accepted engineering or manufacturingtolerance for similar devices, up to, for example, 10% deviation invalue or 10° deviation in angle.

In the semiconductor industry, lithography limitations may beproblematic when scaling down integrated circuits. Issues that may ariseinclude current supply system resistance impacts on performance,electromigration reliability on lower levels, reduced wireability due towiring resources applied to power and ground and lithography impacts dueto non-uniform track images.

One solution to lithography limitations may include scaling downcircuits and building the devices in a vertical dimension (i.e., in athird dimension). Scaling down circuits may include using buriedcontacting layers below (i.e., opposite to the wiring stack) the activearea supplying power and ground. For example, the ground (GND) is belowthe n-channel FET (nFET) area and the voltage source (VSS) is below thep-channel FET (pFET) area. An advantage may be provided by having thepower and the ground supplied from opposite ends of the wiring stack.Advantages may also include an optimized density, no electromigration,more space for signal wires, accessibility to signal pins, providinguniform pitch on lower metal and vertical interconnect access (VIA)layers, and less VIAs.

An additional advantage may include avoiding interference between thesignal lines and the power supply rails of the integrated circuit. Thismay allow for a greater flexibility for placing the logic cells of theintegrated circuit.

FIGS. 1a-6a show circuit diagram elements of a NAND gate and FIGS. 1b-6bshow the corresponding elements of a physical structure of an integratedcircuit 100 that include multiple adjacent layers. For a betterunderstanding, the integrated circuit 100 is shown with an increasingnumber of elements and in some of the figures, some elements are hiddenbehind other elements (i.e., can be seen from differing angles notnecessarily shown in each figure). FIGS. 1a-6a correspond to FIGS. 1b-6bin terms of electrical circuit diagrams. In the following description,reference is made primarily to the physical structure as shown in FIGS.1b -6 b.

Referring to FIGS. 1 a/b, the integrated circuit 100 includes a stack110 of a first field effect transistor (FET) 110 a and a second FET 110b. Both FETs 110 a, 110 b each include a channel region. The channelregion of the first FET 110 a is covered by a gate dielectric 112 and agate 113. The channel regions of the FETs 110 a and 110 b of the stack110 are electrically connected in series. The stack 110 extends in adirection V which is substantially perpendicular to the layers of theintegrated circuit 100. The two FETs 110 a, 110 b of the stack 110 areof the same type. For example, both FETs of the stack 110 may ben-channel FET (nFETs) or both FETs of the stack 110 may be p-channelFETs (pFETs). The two opposed ends of the stack 110 of FETs 110 a, 110 bcorrespond to a power support end 111 and a signal end 114.

The terms substantially, substantially similar, about, or any other termdenoting functionally equivalent similarities refer to instances inwhich the difference in length, height, or orientation convey nopractical difference between the definite recitation (e.g. the phrasesans the substantially similar term), and the substantially similarvariations. In an embodiment, substantial (and its derivatives) denote adifference by a generally accepted engineering or manufacturingtolerance for similar devices, up to, for example, 10% deviation invalue or 10° deviation in angle.

The channel regions of the stack 110 (i.e., the channel regions of thefirst FET 110 a and the second FET 110 b) may be vertical channelregions. Vertical channel regions may refer to channel regions in whichcharge carriers may move in a direction substantially orthogonal to thelayers of the integrated circuit 100 in a conductive state of the firstFET 110 a and/or second FET 110 b.

The first FET 110 a and the second FET 110 b may include more than onevertical channel region. In the embodiment shown in FIG. 1b , the stack110 is shown as a single column having a circular cross section.However, the stack 110 may also include a bundle of vertical columns,wherein the channel regions are each covered by a gate dielectric andthe gate dielectrics are surrounded by a common gate.

Cross sections of vertical channel regions may have a longitudinal axisC_(L) and a transversal axis C_(T). As shown in FIG. 1b , thelongitudinal axis C_(L) and the transversal axis C_(T) may havesubstantially the same length. In the embodiment shown in FIG. 1b , thecross sections are approximately circular cross sections. Further crosssections with a longitudinal axis C_(L) and transversal axis C_(T) ofapproximately the same length may be quadratic cross sections, hexagonalcross sections and/or triangular cross sections.

Alternatively, vertical channel regions may have an elongated crosssection, wherein the longitudinal axis C_(L) is longer than thetransversal axis C_(T). For example, the cross section may have arectangular or elliptical shape.

The stack 110 may include a common nanowire, wherein the common nanowireincludes the channel regions of the FETs 110 a and 110 b of the stack110. However, the stack 110 may also have a shape different from ananowire.

An embodiment shown in FIG. 1b , the first FET 110 a and the second FET110 b are both gate-all-around FETs. The gates of the FETs 110 a and 110b completely surround the channel regions of the FETs 110 a and 110 b.This may improve the electrical characteristics of the FETs 110 a, 110b.

In addition, the integrated circuit 100 includes a second pair of FETs115, 116, which may be formed as vertical structures different from thestack 110. The FET 115 may also be formed as a different verticalstructure than the FET 116. Thus, the stack 110, the FET 115 and the FET116 may form three different columns extending in the direction V. Thelower ends of the FETs 115 and 116 may be considered as power supportends.

As shown in FIGS. 1a-1b , the gate 113 of the first FET 110 a and thegate of the FET 116 may be provided in the same layer of the integratedcircuit 100.

Referring now to FIGS. 2a /2 b, the gate 113 of the first FET and thegate of the FET 116 may be electrically connected with a conductiveelement 153. The conductive element 153 may be provided in the samelayer as the gate 113 of the first FET 110 a and the gate of the FET116. In particular, the gate 113 of the first FET 110 a, the conductiveelement 153 and the gate of the FET 116 may be formed as one piece of asingle material. Alternatively, the gate 113 of the first FET 110 a andthe gate of the FET 116 may be formed of different materials. Forexample, the material of the gate of the first FET 110 a and the gate ofthe FET 116 may be selected to obtain a certain threshold voltage of thefirst FET 110 a and the FET 116. The material of the conductive element153 may be selected to have a particularly low resistivity.

Referring now to FIGS. 3a /3 b, the gate of the FET 115 may beelectrically connected to the gate of the second FET 110 b withconductive elements 151, 152. Therefore, the integrated circuit 100 mayinclude a conductive element 151 extending in the same layer as the gateof the second FET 110 b. The conductive element 151 and the gate of thesecond FET 110 b may be formed as one piece. This may facilitatemanufacturing the integrated circuit 100. They may also be formed ofdifferent materials as explained with respect to gate 113 and conductiveelement 153 above. Another conductive element 152 may be provided in thesame layer as the gate of the FET 115. Said layer may correspond to thelayer in which the gate 113 of the first FET 110 a, the gate of the FET116 and the conductive element 153 are formed. The gate of the FET 115and the conductive element 152 may be formed as one piece. They may alsobe formed of different materials as explained with respect to gate 113and conductive element 153 above. The vertical conductive element 142may electrically connect the conductive element 151 and the conductiveelement 152.

Vertical conductive elements of the integrated circuits disclosed inthis specification may be formed of a metal. Metals may have aparticulary high conductivity. Moreover, metals may facilitate heatdistribution within the integrated circuit. Thermal hotspots may beavoided. Alternatively, vertical conductive elements of the integratedcircuit may be formed of a semiconductor material. Vertical conductiveelements may be formed of the same semiconductor material as a sourceregion, a channel region or a drain region of a FET of the integratedcircuit. Using the same semiconductor material for the verticalconductive elements and the FETs may avoid contamination of the FETswith materials which could deteriorate the electrical characteristic ofthe FETs. Moreover, it may be possible to use the same process forforming the vertical conductive elements and the vertical channelregions of the FETs. Physical dimensions (e.g., a cross section, awidth, a length or a height) of the vertical conductive elements and theFETs may be the same. Patterning during manufacture of the integratedcircuit may be easier if the number of features having differentdimensions is reduced.

Referring now to FIGS. 4 a/b, the integrated circuit 100 may include aninternal connection element 160 connecting the FET 115 and the FET 116.The internal connection element 160 may extend in a single layer of theintegrated circuit 100. Charge carriers may move in the internalconnection element 160 substantially parallel to the layers of theintegrated circuit 100. The internal connection element 160 electricallyconnects the upper ends of the FETs 115 and 116.

Internal connection elements of the integrated circuits disclosed inthis specification may be formed of a metal. Alternatively, the internalconnection elements of the integrated circuit may be formed of asemiconductor material. In particular, the internal connection elementsof the integrated circuit may be formed of the same semiconductormaterial as the material of source and/or drain regions of the FETs ofthe integrated circuit. This may avoid contact resistances between thesource and/or drain regions of the FETs of the integrated circuit andthe internal connection elements.

The integrated circuit 100 may further include a vertical conductiveelement 143 and a vertical conductive element 144. The verticalconductive element 143 may be electrically connected with the conductiveelement 153. The vertical conductive element 143 may extend from theconductive element 153 in an upward direction. The vertical conductiveelement 144 may be electrically connected with the internal connectionelement 160. The vertical conductive element 144 may extend from theinternal connection element 160 in an upward direction.

Referring now to FIGS. 5a /5 b, the vertical conductive element 143 maybe electrically connected to a first input 171 of the integrated circuit100. Thus, a gate of the FET 110 a (FIGS. 1-2) of the stack 110 (FIGS.1-2) and a gate of the FET 116 (FIGS. 1-3) are connected to each otherand may correspond to a first input 171 of the integrated circuit 100.

Moreover, the gate of the FET 110 b (FIGS. 1-2) and the gate of the FET115 (FIGS. 1-2) extend in different layers of in the integrated circuit100. The gate of the second FET 110 b and the gate of the FET 115 areconnected to each other via the conductive element 151, the verticalconductive element 142 and the conductive element 152 and further via anadditional vertical conductive element 141 (FIG. 4) to a second input172 of the integrated circuit 100. An end of the stack 110 maycorrespond to a signal end 114. The signal end 114 of the stack 110 maybe connected to an output 181 of the integrated circuit 100. Thevertical conductive element 144 (FIG. 4) may provide an electricalconnection from the internal connection element 160 (FIG. 4) to theoutput 181 of integrated circuit 100.

Referring now to FIGS. 6a /6 b, the power support end 111 of the stack110 may be connected to a power supply rail 191 of the integratedcircuit 100. A power support end of the FET 115 (FIGS. 1-2) and a powersupport end of the FET 116 (FIGS. 1-2) may be both connected to anotherpower supply rail 192. The power supply rail 191 and the power supplyrail 192 may be provided parallel to each other.

The electrically connected elements of the integrated circuit 100 may beconsidered a logical cell 100, in particular a NAND gate 100 with inputs171 and 172 and an output 181. The inputs 171 and 172 and the output 181may be considered as signal lines of the integrated circuit. Additionallocal and global signal wiring layers on top of the shown input andoutput layer may exist.

As shown in the integrated circuit 100 (i.e., FIGS. 1b-6b ) the channelregions of the FETs 110 a, 110 b, 115, 116 of the integrated circuit 100may be provided between a layer that includes the signal lines 171, 172,181 and a layer that includes the power supply rails 191, 192. Thus,interferences between the signal lines and the power supply rails of theintegrated circuit may be avoided. This may allow for a greaterflexibility for placing the logic cells of the integrated circuit.

Using FETs with vertical channel regions to form the integrated circuitof FIGS. 1-6 may reduce the need for additional vertical connectionelements, because the vertical channel regions already provide someelectrical connections between the different layers of the integratedcircuit.

The power supply rail 191 may provide a ground voltage GND and the powersupply rail 192 may provide a high potential voltage VDD for driving theintegrated circuit 100. The FETs 110 a and 110 b of the stack 110 may benFETs and the FETs 115 and 116 may be pFETs.

FETs of the disclosed integrated circuits may be provided in regularintervals in the extension direction of the power supply rails. Thedistance between the FETs in the extension direction of the power supplyrails and the distance between the FETs in a direction perpendicular tothe extension direction of the power supply rails may be equal. Such aregular arrangement of the FETs may facilitate reliably manufacturingthe integrated circuit. Moreover, a regular arrangement of FETs mayallow for optimizing the density of FETs of the integrated circuit.

FIGS. 7a-7d shows an integrated circuit 200 that corresponds to theintegrated circuit 100 described hereinbefore. FIG. 7a is a circuitdiagram of a balanced NAND gate, FIG. 7b shows the physical structure ofthe balanced NAND gate, FIG. 7c shows vertical structures on nodes of agrid and FIG. 7d further illustrates FIG. 7 c.

Referring now to FIGS. 7a, 7b , an integrated circuit 200 that includesmultiple adjacent layers in accordance with an embodiment of the presentinvention. The integrated circuit 200 corresponds to the integratedcircuit 100 described hereinbefore. In addition to a stack 210 includinga first FET 210 a and a second FET 210 b corresponding to the firststack 110 of the integrated circuit 100, the integrated circuit 200includes an additional stack 217 including a third FET 217 a and afourth FET 217 b.

Power support ends 211 and 218 of both stacks 210 and 217 may beconnected to a power supply rail 291. Signal ends 214 and 219 of bothstacks 210 and 217 may be connected to an output 281 of the integratedcircuit 200. Moreover, the gate of the first FET 210 a, the gate of thethird FET 217 a and a connection element 253 may extend in the samelayer of the integrated circuit. In particular, the gate of the firstFET 210 a, the gate of the third FET 217 a and the connection element253 may be formed as one piece. The gate of the second FET 210 b and thegate of the fourth FET 217 b may also extend in a single layer togetherwith a connection element 251.

The resistance of a stack of two FETs may be larger than the resistanceof a vertical structure that may only include one FET. Thus, providingan additional stack 217 electrically connected in parallel to the stack211 may result in an integrated circuit 200 having a more balancedcharacteristic compared to the integrated circuit 100. In particular,the impedance at the output 281 of the integrated circuit 200 may beless dependent on the voltage provided at the output 281 of theintegrated circuit 200.

FIGS. 7c and 7d illustrate that the vertical structures of theintegrated circuit 200, in particular the vertical structures 241, 210,217 of the integrated circuit 200, are arranged above nodes N of avirtual two-dimensional regular grid G. The grid G has a translationalsymmetry. Moreover, the grid G has a rotational symmetry as turning thegrid by 90° leads to the very same grid. The longitudinal spacing G_(L)between two nodes of the grid in a longitudinal direction is equal tothe transversal spacing G_(T) between two nodes of the grid in atransversal direction, wherein the longitudinal direction isperpendicular to the transversal direction.

FIGS. 8a-13a show the circuit diagram elements of a Logic NOR gate andFIGS. 8b-13b show the corresponding elements of a physical structure ofan integrated circuit 300 that include multiple adjacent layers. Theintegrated circuit 300 is shown with an increasing number of elements.FIGS. 8a-13a correspond to FIGS. 8b-13b in terms of electrical circuitdiagrams. In the following description, reference is made primarily tothe physical structure as shown in FIGS. 8b -13 b.

Referring now to FIG. 8b , the integrated circuit 300 includes a stack310. The stack 310 includes a first field effect transistor (FET) 310 aand a second FET 310 b. The first FET 310 a includes a channel regioncovered by a gate dielectric 312 and a gate 313. A channel region of thesecond FET 310 b is electrically connected in series with the channelregion of the first FET 310 a. The first FET 310 a and the second FET310 b are stacked in a direction V which is substantially perpendicularto the layers of the integrated circuit 300. The two FETs 310 a and 310b are of the same type. For example, both FETs of the stack 310 may bep-channel FETs, pFETs, or both FETs 310 a, 310 b of the stack 310 may ben-channel FETs, nFETs. A first end of the stack 310 may be considered aspower support end 311 and the other end at the opposite side as signalend 314.

Charge carriers may move in the channel regions of the first FET 310 aand the second FET 310 b (i.e., in the channel regions of the stack 310)in a direction, which is substantially perpendicular to the layers ofthe integrated circuit 300, if the channels are open (i.e., in aconductive state of the first FET 310 a and the second FET 310 b). Thus,the channel regions may be considered as vertical channel regions.

According to FIG. 8b , the stack 310 is shown as one pillar. The crosssection of the pillar corresponds to a circle. In an alternativeembodiment, the stack 310 may include several pillars each having avertical channel region. The vertical channel regions may each becovered by a gate dielectric (e.g., a gate oxide) and the gatedielectrics may be surrounded by a common gate. Typically, the pillarswill have the same cross section to facilitate manufacturing of theintegrated circuit. It is also conceivable that the pillars havedifferent cross sections to allow for a better fine tuning of theelectrical characteristics of the FETs.

The vertical channel regions may have cross sections having alongitudinal axis C_(L) and a transversal axis C_(T). As shown in FIGS.8b-13b , the longitudinal axis C_(L) may be as long as the transversalaxis C_(T). Cross sections with approximately equal longitudinal axisC_(L) and transversal axis C_(T) may be, for example, circles, quadrats,triangles and hexagons.

In an embodiment, the longitudinal axis C_(L) of the cross section ofthe vertical channel regions may be longer than the transversal axisC_(T). Such a cross section may be called elongated cross section.Rectangles or ellipses may be examples for elongated cross sections.

The stack 310 may be formed from a common nanowire, wherein the commonnanowire includes the channel regions of the FETs 310 a and 310 b of thestack 310. In other embodiments, the shape of the stack 310 may differfrom a nanowire.

The gate of the FETs shown in FIGS. 8b-13b , in particular the first FET310 a and the second FET 310 b, may be gate-all-around FETs. The gatesof the FETs completely surrounded the channel regions of the FETs.

According to FIG. 8b , a second pair of FETs 315, 316 is provided in theintegrated circuit 300. The FET 315 and the FET 316 may be formed asdifferent vertical structures of the integrated circuit 310. The FETs315, 316 may also each be formed as vertical structures different fromthe stack 310. A single layer of the integrated circuit 300 includesboth the gate 313 of the first FET 310 a and the gate of the FET 316.

As shown in FIG. 9b , a conductive element 351 provided in the samelayer that includes the gate 313 (shown in FIGS. 8a-13a, 8b ) of thefirst FET 310 a (shown in FIGS. 8a, 8b ) and the gate of the FET 316 mayelectrically connect the gate 313 of the first FET 310 a and the gate ofthe FET 316. The gate 313 of the first FET 310 a, the conductive element351 and the gate of the FET 316 may be formed as one piece.

The gate of the second FET 310 b (shown in FIGS. 8a, 8b ) is provided ina different layer of the integrated circuit 300 than the gate of the FET315. As shown in FIG. 10b , the gate of the FET 310 b is electricallyconnected to the gate of the FET 315. The electrical connection may beestablished by the depicted conductive element 353, the verticalconductive element 344 and the conductive element 352. The conductiveelement 353 and the gate of the FET 315 may be formed as one part.Alternatively or in addition, the gate of the FET 310 b and theconductive element 352 may also be formed as one piece.

As shown in FIG. 11b , an internal connection element 360 may provide anelectrical connection between the FET 315 and the FET 316. The internalconnection element 360 is provided in a single layer of the integratedcircuit 300. Electrical currents may flow substantially parallel to thelayers of the integrated circuit 300 in the internal connection element360. The internal connection element 360 may in particular connect theupper ends of the FETs 315 and 316. The lower ends of the FETs 315 and316 may be considered as power support ends.

FIG. 12b shows a vertical conductive element 343 electrically connectingthe internal connection element 360 to an output 381 of the integratedcircuit 300. The output 381 is furthermore electrically connected to thesignal end 314 of the stack 310.

A vertical conductive element 341 electrically connects the conductiveelement 351 to an input 372 of the integrated circuit 300. Anothervertical conductive element 342 provides an electrical connectionbetween the conductive element 352 and another input 371 of theintegrated circuit 300.

According to FIG. 13b , the power support end of the stack 310 isconnected to a power supply rail 392 and the power support ends of theFETs 315 and 316 are connected to another power supply rail 391.

The power supply rail 391 may provide a ground voltage GND and the powersupply rail 392 may provide a high potential voltage VDD for driving theintegrated circuit 300. The FETs 310 a, 310 b (shown in FIGS. 8a, 8b )of the stack 310 may be pFETs and the FETs 315 and 316 may be nFETs.

The electrically connected elements of the integrated circuit 300correspond to a logical cell, in particular a NOR gate.

The physical structure of the NOR gate 300 may be mirror-symmetric tothe physical structure of the NAND gate 100, wherein the power supplyrails 191 (shown in FIGS. 6a, 6b ) and 391 provide both a ground voltageGND, wherein the power supply rails 192 (shown in FIGS. 6a, 6b ) and 392provide both a high potential voltage VDD, and wherein the type of theFETs 110 a, 110 b (shown in FIGS. 1-2 a and 1-2 b), 115 (shown in FIGS.1a-6a, 1b-2b ) and 116 (shown in FIGS. 1a-5a, 1b-5b ) is opposite to thetype of the FETs 310 a, 310 b and 315, 316, respectively.

An integrated circuit may include a sub-circuit corresponding to theintegrated circuit 100 and a sub-circuit corresponding to the integratedcircuit 300, which share the power supply rails 191/391 and 192/392.This may be an example of an integrated circuit that includes multipleadjacent layers, wherein the integrated circuit includes a (first) stack110 of two FETs of the same type and second stack 310 of two FETs of thesame type, which is different than the type of the FETs of the (first)stack 110, wherein a power end of the (first) stack 110 is connected toa power supply rail 191/391 and a power end of the second stack 310 isconnected to another power supply rail 192/392.

Extending from NAND and NOR, as shown as an example of logic cells,multiple combinations of p- and n-channel stacks and p- and n-channelsingle devices may be made with p-channels located above or below VDDrails and n-channels located above or below GND rails. Using themultiple variations of p- and n-channel stacks and devices, the devicesmay be connected to appropriately form the desired function with theconnection elements shown in the NAND and NOR examples.

FIGS. 14a-20a show circuit diagram elements of a SRAM Cell and FIGS.14b-20b show the corresponding elements of a physical structure of anintegrated circuit 400 that include multiple adjacent layers. For abetter understanding, the integrated circuit 300 is shown with anincreasing number of elements. FIGS. 14a-20a correspond to FIGS. 14b-20bin terms of electrical circuit diagrams. In the following description,reference is made primarily to the physical structure as shown in FIGS.14b -20 b.

Referring now to FIGS. 14b-20b , elements of a physical structure of anintegrated circuit 400 having a layered structure. In particular, FIGS.14b-20b disclose elements of a SRAM cell 400. The integrated circuit 400is shown with an increasing number of elements from FIG. 14b to FIG. 20b. FIGS. 14a-20a correspond to FIGS. 14b-20b in terms of electricalcircuit diagrams. The following description focuses on the physicalstructure of the integrated circuit 400 and the electrical circuitdiagrams are mainly provided for a better understanding of the functionof the respective elements.

As shown in FIG. 14b the integrated circuit 400 includes a (first) stack410 of a first field effect transistor (FET) 410 a and a second FET 410b. The first FET 410 a and the second FET 410 b each include a channelregion. The channel regions of the first FET 410 a and the second FET410 b are electrically connected in series. The (first) stack 410extends in a vertical direction V that is substantially perpendicular tothe layers. The first FET 410 a and the second FET 410 b of the stack410 are of the same type. The two opposed ends of the stack 410correspond to a power support end 411 and a signal end 414.

The integrated circuit 400 may correspond to a SRAM cell 400. The SRAMcell 400 includes a memory element 470 as shown in FIG. 20a . The memoryelement 470 (shown in FIG. 20a ) includes a first inverter 471 (shown inFIG. 20a ) that includes a first inverter FET 410 a. The first inverterFET 410 a is shown as first FET 410 a of the (first) stack 410 in FIG.14b and is connected to a power supply rail 491 (shown in FIGS. 20a, 20b). The SRAM cell 400 further includes an access FET 410 b electricallyconnected to a bitline 481 (shown in FIGS. 20a, 20b ). The access FET410 b is shown as second FET 410 b of the (first) stack 410 in FIG. 14b.

As shown in FIG. 14b , the integrated circuit/SRAM cell 400 includes asecond stack 415 that may include two FETs 415 a and 415 b. The channelregions of the two FETs 415 a and 415 b are electrically connected inseries. In particular, a source region of one of the two FETs 415 a and415 b may correspond to a drain region of the other one of the two FETs415 a and 415 b. A lower end of the stack 415 may be called a powersupport end and an upper end of the stack 415 may be called a signal endof the stack 415.

The channel regions of the (first) stack 410 and the second stack 415are provided in two layers between the layer that includes the powersupply rail 491 (shown in FIGS. 20a, 20b ) and layers that includesignal lines, in particular the bitline 481 and the bitline 482 of theintegrated circuit 400 as shown in FIG. 20b . The bitline 482 may alsobe considered as a complementary bitline 482, because the bitlines 481and 482 are configured for transmitting complementary data. Theseparation of the layer that includes the gates of the FETs 410 a and415 a and the layer that includes the gates of the FETs 410 b and 415 bfrom the layer that includes the power supply rails 491 and 492 and thelayer that includes the bitlines 481 and 482 may substantially reducethe routing complexity of the integrated circuit 400. In particular,less VIA contacts may be required and shorter wires may be sufficientfor manufacturing the integrated circuit. This may increase the devicedensity of an integrated circuit. Moreover, it may reduce powerconsumption induced by wire capacitances.

The channel regions of the stacks 410 and 415 shown in FIG. 14b arevertical channel regions. In vertical channel regions, charge carriersmay move in a direction substantially perpendicular to the layers of theintegrated circuit 400 in a conductive state of the respective FETs.

The first inverter 471 shown in FIG. 20a further includes a secondinverter FET 417 that includes a gate. The second inverter FET 417 is ofa different type than the first inverter FET 410 a (shown in FIGS. 14-17a, 14-20 b) that includes a gate. An end of the second inverter FET 417is connected to a further power supply rail 492. The second inverter 472also includes two FETs, namely the FET 415 a (shown in FIGS. 14-17 a,14-20 b) and the FET 416. The FET 416 is hidden behind the stack 410 inFIG. 14b . The stack 410, the stack 415, the FET 417 and the FET 416 maybe arranged in corners of a rectangle, when seen from above. Inparticular, the stack 410, the stack 415, the FET 417 and the FET 416may be provided in corners of a quadrat.

As shown in FIG. 15b , the gate 413 of the first inverter FET 410 a andthe gate of the second inverter FET 417 are electrically connected witheach other by a conductive element 451 extending within a single layerof the integrated circuit 400. In particular, the gate of the firstinverter FET 410 a, the gate of the second inverter FET 417 and theconductive element 451 may be formed as one piece to facilitatemanufacturing of the integrated circuit 400.

Correspondingly, a conductive element 452 is provided, which connectsthe gate of the FET 415 a and the gate of the FET 416. The conductiveelement 452 extends within a single layer of the integrated circuit 400.The gate of the FET 415 a, the gate of the FET 416 and the conductiveelement 452 may be formed as one piece, too.

In an embodiment shown in FIGS. 14b-20b , the first inverter FET 410 aand the access FET 410 b are n-channel FETs, nFETs, and the secondinverter FET 417 is a p-channel FET, pFET. Correspondingly, the FET 415a and the FET 415 b are nFETs and the FET 416 is a pFET. For givenphysical dimensions, nFETs may have a higher conductivity. Using FETs ofthe type with the higher conductivity may be particularly usefull whenthe FETs are connected in series to reduce the power consumption of theintegrated circuit. However, embodiments are conceivable in which thefirst inverter FET and the access FET are pFETs and the second inverterFET is a nFET. This may be usefull for an integrated circuit withinverted logic.

As shown in FIGS. 16b and 17b , the integrated circuit 400 includes anintermediate layer. The intermediate layer is arranged between a firstgate layer that includes the gate 413 of the first inverter FET 410 aand a second gate layer that includes the gate of the access FET 410 b.The memory element 470 (shown in FIG. 20a ) includes a second inverter472 (shown in FIG. 20a ). The second inverter 472 is formed with the FET415 a and the FET 416.

The second inverter 472 is cross-coupled with the first inverter 471(shown in FIG. 20a ) by means of two cross-coupling conductors. As shownin FIG. 17b , one cross-coupling conductor includes the conductiveelement 462 (shown in FIGS. 16-20 a) and the vertical conductive element441 (shown in FIGS. 16-20 a). The vertical conductive element 441provides a connection between the upper end of the FET 415 a and theconductive element 451 (shown in FIGS. 15-17 a, 20 a and 15-20 b). Theother cross-coupling conductor includes the conductive element 461 and afurther vertical conductive element 442 (hidden behind the conductiveelement 461 in FIG. 17b ), which connects the conductive element 461with the conductive element 452. Thus, the cross-coupling conductorsboth include a section 461, 462 extending within the intermediate layer.

According to FIG. 18b , the access FET 410 b includes a gate, whereinthe gate is connected to a wordline 453. The gate of the FET 415 b maybe connected to the wordline 453, too. The wordline 453 may extend inthe same layer as the gate of the access FET 410 b. In particular, thewordline 453 and the access FET 410 b of the SRAM cell 400 may be formedas one piece. This may further reduce the manufacturing complexity ofthe integrated circuit 400.

As shown in FIG. 19b , the signal ends of the stacks 410 and 415 areconnected to respective bitlines 482 and 481.

The power support ends of the stacks 410, 415 are connected to the powersupply rail 491 and the power support ends of the FETs 416 and 417 areconnected to the power supply rail 492 as shown in FIG. 20 b.

In the embodiment shown in FIGS. 13b-20b , the width of the wordline 453in a direction PT perpendicular to the extension direction of the powersupply rail 491 is only slightly larger than the gate of the access FET410 b. In other embodiments, a wider wordline 453 may be provided whichalso extends above the FETs 416 and 417. The wordline 453 mayelectrically connect the access FETs of a plurality of SRAM cellsprovided in a direction P_(L) along the power supply rails 491 and 492.Thus, the wordline 453 may allow for reading or writing data from/to aplurality of SRAM cells in parallel. Theretofore, the wordline 453 mayextend parallel to the power supply rail 491. More SRAM cells may beaddressed by a wider wordline 453.

The bitline 481 extends perpendicular to the power supply rail 491. Thebitline 481 may connect a plurality of SRAM cells provided in adirection PT transversal to the extension direction P_(L) of the powersupply rail 391. The other bitline 482 may be provided parallel to thebitline 481.

The wordline 453, the bitline 481/482 and the power supply rail 491/492may be provided in different layers of the integrated circuit 400. Thus,a larger two dimensional array of SRAM cells may be provided withoutspecial bridging structures for insulating crossing wordlines/bitlines.

According to an embodiment shown in FIGS. 21 and 22, an integratedcircuit 500 that includes multiple adjacent layers may be provided whichincludes mirror-symmetrical SRAM cells 501 and 502. The SRAM cells 501and 502 may correspond to the SRAM cell of the integrated circuit 300.The plane of symmetry of the two mirror-symmetrical SRAM cells 501 and502 may extend perpendicular to the layers of the integrated circuit 500and parallel to the power supply rails 591, 592, 593. Twomirror-symmetrical SRAM cells 501 and 502 may share a power supply rail591.

For example, the two mirror-symmetrical SRAM cells 501 and 502 may sharethe power supply rail 591 being connected to the stacks of the SRAMcells 501 and 502.

In addition, the two mirror-symmetrical SRAM cells 501 and 503 (SRAMcell 503 is only partly depicted in FIG. 21) may share a power supplyrail 593 being connected to the second inverter FETs of the SRAM cells501 and 503.

Thus, wider power supply rails 591, 592, 593 may be used for driving theSRAM cells. This may mitigate voltage fluctuations when reading/writingSRAMs cells connected to a common wordline. In addition, the distancebetween the SRAM cells in a direction PT transverse to the extensiondirection P_(L) of the power supply rails 591, 592 may be reduced.

The integrated circuit 500 includes a pair of two further SRAM cells504, 505. The pair of the two further SRAM cells 504, 505 is atranslated version of the pair of SRAM cells 501, 502. An array of SRAMcells may be formed by providing even further translated SRAM cellsalong the power supply rails 591, 592, 593 and by continuing to providemirror-symmetric SRAM cells in a direction perpendicular to the powersupply rails 591, 592, 593.

As shown in FIG. 22, the wordline 553 may be used to address the SRAMcells 501 and 504. Correspondingly, the wordline 554 may be used toaddress the SRAM cells 502 and 505. The wordline 553 may extend onlyalong the vertical structures that include two FETs and not above thevertical structures that include only one FET. The SRAM cells may beread/written with a bitline 581, a complementary bitline 582, a bitline583 and a complementary bitline 584. Thus, an integrated circuit withalternating bitlines and complementary bitlines may be provided.

FIG. 23 shows another integrated circuit 600 which is very similar tothe integrated circuit 500 shown in FIGS. 21 and 22. The features 684,683, 682, 681, 603, 693, 604, 605, 601, 691, 602 and 692 correspond tothe features 584, 583, 582, 581, 503, 593, 504, 505, 501, 591, 502 and592 (shown in FIG. 22). However, the wordlines 653 and 654 differ fromthe wordlines 553 and 554 (shown in FIG. 22). In particular, thewordlines 653 and 654 also extend above the vertical structures thatinclude only one FET. The wider wordlines 653, 654 may allow foraddressing more SRAM cells.

FIG. 24 shows a still further integrated circuit 700. The integratedcircuit with its features 703, 793, 704, 705, 701, 791, 702, 792substantially corresponds to the integrated circuit 500 shown in FIG.21. However, the SRAM cells 704, 705 are mirror-symmetrical versions ofthe SRAM cells 701, 702 instead of merely translated versions. Thisallows for a configuration, in which two bitlines are followed by twocomplementary bitlines which are again followed by two bitlines.

FIG. 25 illustrates another integrated circuit 800 that include multipleadjacent layers. The integrated circuit includes vertical structuresextending in a vertical direction V that is substantially perpendicularto the layer of the integrated circuit. The vertical structures includea vertical connection element 841, a stack 810 of two FETs that eachinclude a vertical channel region, and a FET 817 that also includes avertical channel region.

The vertical structures 841, 810, 817 are arranged above nodes N of avirtual two-dimensional regular grid G. The grid G has a translationalsymmetry. The longitudinal spacing G_(L) between two nodes of the gridin a longitudinal direction is different from the transversal spacingG_(T) between two nodes of the grid in a transversal direction, whereinthe longitudinal direction is perpendicular to the transversaldirection. However, other embodiments may prescribe that the spacingG_(L) between two nodes of the grid in a longitudinal direction is thesame as the transversal spacing G_(T) between two nodes of the grid in atransversal direction.

FIG. 26 shows a further virtual two-dimensional regular grid G havingnodes N, above which vertical structures may be arranged. The grid G hasa hexagonal structure.

The different elements of the integrated circuit may be embedded in anelectrically insulating material. For example, the “open” spaces may befilled with a semiconductor oxide.

In the embodiments depicted in the FIGS. of the application, the powersupply rails have been shown below and the signal lines above thechannel regions of the FETs. However, a mirrored arrangement is alsoconceivable, wherein the signal lines are provided below and the powersupply rails above the channel regions of the FETs.

The integrated circuit may include a stack of two FETs having the sametype and a stack of two FETs having the same type different from thetype of the first stack. In particular, one stack may belong to a NANDcell and the other stack may belong to a NOR cell. The two stacks may beprovided as vertical structures on different power supply rails.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration but are not intended tobe exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiment, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A microelectronic device comprising a pluralityof layers, the microelectronic device comprising: a pair of transistorsstacked vertically and connected in series, each of the pair oftransistors are of the same type; and a memory element including aninverter comprising a first inverter transistor and an accesstransistor, the first inverter transistor is connected to a power supplyrail, the access transistor is connected to a bitline, wherein the firstinverter transistor is a first transistor of the pair of verticallystacked transistors and the access transistor is a second transistor ofthe pair of vertically stacked transistors, and wherein the pair oftransistors are arranged substantially perpendicular to the plurality oflayers.
 2. The microelectronic device according to claim 1, wherein theinverter further comprises: a second inverter transistor of a differenttype than the first inverter transistor, wherein the second invertertransistor is arranged substantially perpendicular to the plurality oflayers and connected to a further power supply rail.
 3. Themicroelectronic device according to claim 2, wherein a gate of the firstinverter transistor and a gate of the second inverter transistor areelectrically connected to each other by a conductive element extendingwithin a single layer of the microelectronic device.
 4. Themicroelectronic device according to claim 3, wherein the connection tothe gate of the first inverter transistor, the connection to the gate ofthe second inverter transistor and the conductive element are the sameconductive element.
 5. The microelectronic device according to claim 2,wherein the first inverter transistor and the access transistor aren-channel transistors, and wherein the second inverter transistor is ap-channel transistor.
 6. The microelectronic device according to claim1, further comprising: an intermediate layer arranged between a firstgate layer comprising the gate of the first inverter transistor and asecond gate layer comprising the gate of the access transistor, whereinthe memory element comprises a cross-coupling inverter cross coupledwith the inverter by means of a cross-coupling conductor, and whereinthe cross-coupling conductor includes a section extending within theintermediate layer.
 7. A microelectronic device comprising a pluralityof layers, the microelectronic device comprising: a pair of transistorsstacked vertically and connected in series, each of the pair oftransistors are of the same type; and a memory element including aninverter and a cross-coupling inverter, wherein the inverter comprises afirst inverter transistor connected to a power supply rail and a secondinverter transistor connected to a further power supply rail, whereinthe second inverter transistor is a different type than the firstinverter transistor, wherein the first inverter transistor is a firsttransistor of the pair of vertically stacked transistors, wherein a gateof the first inverter transistor and a gate of the second invertertransistor are electrically connected to each other by a singleconductive element extending within a single layer of themicroelectronic device, and wherein the first inverter transistor andthe second inverter transistor are substantially perpendicular to theplurality of layers, and an access transistor, wherein the accesstransistor is connected to a bitline, and a gate of the accesstransistor is connected to a wordline, wherein the access transistor isa second transistor of the pair of vertically stacked.
 8. Themicroelectronic device according to claim 7, further comprising: anintermediate layer arranged between a first gate layer comprising thegate of the first inverter transistor and a second gate layer comprisingthe gate of the access transistor, wherein the memory element comprisesa second inverter cross-coupled with the first inverter by means of across-coupling conductor, and wherein the cross-coupling conductorincludes a section extending within the intermediate layer.
 9. Themicroelectronic device according to claim 7, wherein the bitline extendsperpendicular to the power supply rail.
 10. The microelectronic deviceaccording to claim 7, wherein the wordline extends parallel to the powersupply rail.
 11. The microelectronic device according to claim 7,wherein the wordline extends in the same layer as the gate of the accesstransistor.
 12. The microelectronic device according to claim 7, whereinthe wordline and the gate of the access transistor are electricallyconnected in the same layer.
 13. The microelectronic device according toclaim 7, wherein the wordline, the bitline and the power supply rail areprovided in different layers of the integrated circuit.
 14. A method offorming a microelectronic device comprising a plurality of layers, themethod comprising: forming a pair of transistors stacked vertically andconnected in series, each of the pair of transistors are of the sametype; and forming a memory element including an inverter comprising afirst inverter transistor and an access transistor, the first invertertransistor is connected to a power supply rail, the access transistor isconnected to a bitline, wherein the first inverter transistor is a firsttransistor of the pair of vertically stacked transistors and the accesstransistor is a second transistor of the pair of vertically stackedtransistors, wherein the pair of transistors are arranged substantiallyperpendicular to the plurality of layers.
 15. The method according toclaim 14, wherein the inverter further comprises: a second invertertransistor of a different type than the first inverter transistor,wherein the second inverter transistor is arranged substantiallyperpendicular to the plurality of layers and connected to a furtherpower supply rail.
 16. The method according to claim 15, wherein a gateof the first inverter transistor and a gate of the second invertertransistor are electrically connected to each other by a conductiveelement extending within a single layer of the microelectronic device.17. The method according to claim 16, wherein the connection to the gateof the first inverter transistor, the connection to the gate of thesecond inverter transistor and the conductive element are the sameconductive element.
 18. The method according to claim 15, wherein thefirst inverter transistor and the access transistor are n-channeltransistors, and wherein the second inverter transistor is a p-channeltransistor.
 19. The method according to claim 14, further comprising:forming an intermediate layer arranged between a first gate layercomprising the gate of the first inverter transistor and a second gatelayer comprising the gate of the access transistor, wherein the memoryelement comprises a second inverter cross-coupled with the firstinverter by means of a cross-coupling conductor, and wherein thecross-coupling conductor includes a section extending within theintermediate layer.
 20. A method of forming a microelectronic devicecomprising a plurality of layers, the method comprising: forming a pairof transistors stacked vertically and connected in series, each of thepair of transistors are of the same type; and forming a memory elementincluding an inverter and a cross-coupling inverter, wherein the firstinverter comprises a first inverter transistor connected to a powersupply rail and a second inverter transistor connected to a furtherpower supply rail, wherein the second inverter transistor is a differenttype than the first inverter transistor, wherein the first invertertransistor is a first transistor of the pair of vertically stackedtransistors, wherein the first inverter transistor and the secondinverter transistor are substantially perpendicular to the plurality oflayers, and wherein a gate of the first inverter transistor and a gateof the second inverter transistor are electrically connected to eachother by a single conductive element extending within a single layer ofthe microelectronic device, and forming an access transistor, whereinthe access transistor is connected to a bitline, and a gate of theaccess transistor is connected to a wordline, wherein the accesstransistor is a second transistor of the pair of vertically stacked. 21.The method according to claim 20, further comprising: an intermediatelayer arranged between a first gate layer comprising the gate of thefirst inverter transistor and a second gate layer comprising the gate ofthe access transistor, wherein the memory element comprises a secondinverter cross-coupled with the first inverter by means of across-coupling conductor, and wherein the cross-coupling conductorincludes a section extending within the intermediate layer.
 22. Themethod according to claim 20, wherein the bitline extends perpendicularto the power supply rail.
 23. The method according to claim 20, whereinthe wordline extends parallel to the power supply rail.
 24. The methodaccording to claim 20, wherein the wordline extends in the same layer asthe gate of the access transistor.
 25. The method according to claim 20,wherein the wordline and the gate of the access transistor areelectrically connected in the same layer.